I believe the answer is B.
It says that he made 100 cupcakes so we know that the total is 100. It says he baked 4 more than so that means that we are adding 4 to some value. It also says that he baked 3 times the total of cupcakes he made last week. Since the total for last week is unknown, we can express that as x.
So, your equation would be 3x+4=100.
Hope this helps!